1. Field of the Invention
This invention relates generally to wireless communications and more particularly to interference cancellation in a CDMA receiver.
2. Description of the Related Art
Code Division Multiple Access (“CDMA”) provides an effective communications technique for several users to share a communications channel. Unfortunately, when the channel becomes overcrowded, the conventional CDMA receiver performs poorly and multiple access interference (“MAI”) can severely degrade performance. Although the optimal maximum likelihood receiver in this case is easy to describe, it is nearly impossible to implement.
Various conventional techniques examine interference cancellation at the symbol level. Symbol-level matched filters can provide a sufficient statistic for multi-user detection (“MUD”) in an additive white Gaussian noise channel. This well known result concludes that the optimal user bit estimation procedure can be written at the symbol level. Accordingly, these various conventional MUD approaches use symbol-level estimation and cancellation approaches. However, these symbol-level techniques are only approximations to the optimal estimator, and there is no guarantee that these symbol level approximations fully exploit the signal structure.
Additionally, conventional procedures can involve the following computationally expensive process for canceling interference: (1) interpolating the data for each source (base station) to the sampling lattice of the signature waveform (chip center), (2) computing the bit estimates for each user, (3) synthesizing the entire symbol's binary waveform and (4) interpolating the waveform of the whole symbol back to the sampling grid of the data to perform the cancellation.
Some sample-level approaches have been proposed. One example uses a continuous time (i.e., analog) maximum likelihood estimator (“MLE”) approach, which can be used as continuous decision feedback. This MLE approach can be purposed as a single-stage analog process using filters controlled by relative user power levels. Although relatively easy to implement, these approaches are not a good theoretical match to the interference cancellation problem. To remedy such shortcomings, linear minimum mean squared error (MMSE) techniques, such as those based on standard applications of the Kalman filter and other least-squares generalizations, could be used to reduce un-cancelled interference. These techniques fully couple the users (resulting in large matrix computations) and perform interference cancellations in the innovation term in the filter. Accordingly, they remain quite computationally expensive.
Another issue with regard to interference cancellation is the implementation of RAKE receivers. The RAKE technique, which is used in many conventional CDMA handsets, uses multiple base band correlators to separately process several signal multi-path components. This processing is said to be performed by fingers. Each finger is basically an individual demodulator whose timing is adjusted for a particular multi-path component. During soft-handoff, one of these fingers is actually tuned to a neighboring base station. The output of each finger is typically soft information. The receiver typically combines the soft information from each of the fingers to produce a single log-likelihood ratio for each of the transmitted bits. Various interference cancellation problems remain with RAKE implementations, including interference that is introduced by pilot signals that may or may not correspond to processing that is being performed for a finger.
Still another problem with conventional interference cancellation is the inefficient use of computational resources, which are often limited in applications such as mobile handsets. Interference cancellation techniques and corresponding computational architectures have been found to be too inflexible to take full advantage of the computational capabilities of a system in which cancellation is being performed, at various stages of signal processing.
Thus, there remains a need for more efficient interference cancellation, particularly in receivers that implement RAKE processing, and for more efficient allocation of computational resources in typical applications.